The present invention applies to the culture of any photosynthetic organism, i.e. any life form capable of development and of photosynthesis in a suitable nutritive culture medium, in the presence of solar radiation and of a carbon-rich gas such as carbon dioxide.
Among photosynthetic microorganisms concerned by the invention are more particularly represented aquatic plants such as for example micro-algae, moss protonemas, small macro-algae and isolated cells of multicellular plants. These aquatic plants have interesting properties in the fields, notably of pharmacy, human and animal nutrition, of dermo-cosmetology, of energy and of the environment.
Like for most photosynthetic microorganisms, access to this resource essentially consists in assisted culture in suitable reactors. As light is their main substrate, the culture medium should have an optical interface receiving a light flux. The difficulty of cultivating photosynthetic microorganisms lies in the fact that they themselves form obstacles to the passage of light, which is their main substrate. The growth of the culture will therefore stabilize when light no longer penetrates into the thickness of the culture. This phenomenon is called self-shadowing.
The length of the optical path or “light path length”, allows characterization of the different confinement modes and is defined as being:
the length of the light path from its entry into the culture through a transparent optical interface to as far as an opposite opaque wall; or
the half of the distance separating both transparent optical interfaces when the confinement receives light through two opposite transparent optical interfaces.
This optical path length varies between a few centimeters and a few decimeters and essentially determines the propipeion of biomass per unit time and per optical unit surface (surface propipeivity in g/m2/d) and the concentration of the culture between (in g/L) in the final growth phase. The various confinement modes which are applied for ensuring the culture of small aquatic plants may thus be classified according to this characteristic length.
The photosynthesis reaction is also accompanied by consumption of carbon dioxide (CO2) and by propipeion of oxygen (O2). Excess oxygen inhibits the reaction, while the absence of carbon dioxide interrupts it through lack of substrate to be transformed. A gas/liquid interface should therefore be placed for mass transfers between these gases and the liquid phase. In order to promote these exchanges and to avoid heterogeneities, the culture should be the centre of a mixture intended to renew the organisms at the aforementioned optical interface and also at this gas/liquid interface.